1. Field of the Invention
This invention relates to the electrolytic production of metal containing solutions and, more particularly, to the electrolytic production of a low alpha lead methane sulfonate solution with low free acid and impurities using a specially designed membrane cell.
2. Description of Related Art
Electrochemical dissolution of lead in methane sulfonic acid results in the formation of lead methane sulfonate. This product is currently utilized in the electronics fabrication industry such as in the flip chip packaging technology which uses controlled collapse chip connection (C4 process) to connect electronic components. The electrochemical process equations for making lead methane sulfonate are shown in equations 1-2 below:
Anode Reactions
Pbsolidxe2x86x92Pb2+aqueous+2exe2x88x92xe2x80x83xe2x80x83Eqn 1

Cathode Reaction 
At the anode lead metal dissolves by the loss of two electrons to form lead (II) ions in solution. The lead ions subsequently react with methane sulfonate ions to form the desired lead II methane sulfonate in solution. The formation of lead methane sulfonate in the anode compartment is made possible if a membrane layer, preferably an anion exchange type, partitions the anode compartment from the cathode compartment. In an electrolytic cell with an anion exchange membrane partition, lead II ions that forms at the anode are strongly repelled by the membrane, and hence do not migrate to the cathode electrode surface. Methane sulfonic acid electrolyte is preferentially reduced to its sulfonate ion with the evolution of hydrogen gas at the cathode surface, and the catholyte methane sulfonate ions migrate through the membrane to the anolyte where they react with the lead (II) ions to form the desired lead methane sulfonate solution. The anion exchange membrane chemistry allows only anions to pass through it whereas cationic species are normally repelled by it.
Hsueh W. L. and Wan C. C. (1989 Journal of the Chin. I. Ch. E., Vol. 20, No. 1) demonstrated the electrolytic dissolution of lead in a solution of methane sulfonic acid in a laboratory cell using lead panels (99.5% purity) as an anode and graphite panels as cathode, with the cathode compartment separated from the anode compartment by an anion exchange membrane. The anode and cathode compartments of this cell each contained 500 ml of electrolyte, with the anolyte and catholyte initially containing 1.3% and 50% of methane sulfonic acid respectively.
Electrolytic cells that incorporate ion exchange membranes or porous diaphragms for the dissolution of such metals like lead, tin, etc., in suitable electrolytes have been described. U.S. Pat. No. 5,618,404 issued Apr. 8, 1997 discloses the production of lead and tin sulfonate by the use of an acrylic-based electrolytic cell having an anode chamber of 250 ml capacity, two 100 ml product chambers, and two 324-ml cathode chambers. The anode electrode material was a lead or tin rod (99.9% purity) placed in the center of the anode chamber. Two pieces of titanium sheets, 0.9 dm2 each, served as the cathode electrodes. The lead dissolution process and cell design was such that dissolved metals with low alpha counts were only produced by a simultaneous combination of anion and cation exchange membranes in the cell. Metal sulfonates with elevated alpha counts resulted when only anion exchange membranes were used. U.S. Pat. Nos. 3,795,595 and 3,300,397 using electrolytic cells with an ion exchange membrane or a porous woven material, disclose the production of metallic salts in electrolytes other than a methane sulfonic acid medium and with electrolytic cells having small capacity (xcx9c2.5 L), or utilizing environmentally unfriendly mercury cathodes.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an electrolytic process for making metal solutions in an electrolytic membrane cell.
It is another object of the present invention to provide a process for making lead methane sulfonate and in particular low alpha lead methane sulfonate in an electrolytic membrane cell.
A further object of the invention is to provide a membrane cell for electrolytic production of metal containing solutions.
It is yet another object of the present invention to provide a membrane cell for electrolytic production of lead methane sulfonate and low alpha lead methane sulfonate solutions.
Another object of the present invention is to provide metal containing solutions and in particular high concentration low impurity lead methane sulfonate and low alpha lead methane sulfonate solutions.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in the art, are provided in the present invention which, in one aspect, is directed to a method for electrolytically making a metal containing solution and, in particular, a lead methane sulfonate solution, e.g., low alpha lead methane sulfonate solution, comprising the steps of:
providing an electrolytic cell comprising a tank having opposed walls and a bottom, a consumable lead source as an anode, a spaced apart cathode, a membrane, preferably anionic, separating the anode and the cathode forming an anode chamber and a cathode chamber, the membrane having a top edge, bottom edge and opposed side edges and being held along the bottom edge and opposed side edges between a membrane holder comprising a female member comprising opposed vertical members and a connecting lower member the members having an outer peripheral edge and an inner peripheral edge forming an open space between the inner peripheral edges, and the vertical members and connecting lower member having openings for securing a bolt or other fastener, and preferably the openings are only partly therethrough, and a mating male member comprising corresponding opposed vertical members and a connecting lower member, the members having an outer peripheral edge and an inner peripheral edge forming an open space between the inner peripheral edges, and the vertical members and connecting lower members having openings therein, preferably through openings, corresponding to the openings in the female member, with the female member and male member being secured together by fasteners, e.g., bolts, extending through the openings in the male member and into the female member openings wherein either the outer peripheral edge of the male member or female member, preferably the male member, forms a watertight seal with the opposed side walls and tank bottom, e.g., is integrally secured to the opposed side walls and tank bottom;
supplying methane sulfonic acid or other electrolyte acid to the anode chamber and the cathode chamber;
supplying a current between the anode and the cathode to electrolytically dissolve the lead anode forming lead ions in the anode chamber and ionizing the methane sulfonic acid in the cathode chamber to form methane sulfonic acid ions;
continuing the current preferably with a controlled voltage or a controlled current until the desired concentration of lead methane sufonate solution is obtained; and
removing the lead methane sulfonate solution from the tank.
In another aspect of the invention an electrolytic cell is provided for making metal containing solutions and in particular, a lead methane sulfonate solution, e.g., a low-alpha lead methane sulfonate solution comprising:
a tank having opposed walls and a bottom;
a consumable lead source as an anode;
a spaced apart cathode;
an energy source for applying an electric current between the anode and the cathode; and
a membrane, preferably anionic, separating the anode and the cathode forming an anode chamber and a cathode chamber, the membrane having a top edge, bottom edge and opposed side edges and being held along the bottom edge and opposed side edges between a membrane holder comprising a female member comprising opposed vertical members and a connecting lower member, the members having an outer peripheral edge and an inner peripheral edge forming an open space between the inner peripheral edges, and the vertical members and connecting lower members having openings for securing a bolt or other fasteners, and preferably the openings are only partly therethrough, and a mating male member comprising corresponding opposed vertical members and a connecting lower member, the members having an outer peripheral edge and an inner peripheral edge forming an open space between the inner peripheral edges, and the vertical members and connecting lower members having openings therein, preferably through openings corresponding to the openings in the female member, with the female member and the male member being secured together by fasteners, e.g., bolts, extending through the openings in the male member and into the female member openings wherein either the outer peripheral edge of the male member or female member, preferably the male member, forms a watertight seal with the opposed side walls and tank bottom, e.g., is integrally secured to the opposed walls and tank bottom.
In another aspect of the invention a plurality of female members and/or male members are fixedly secured to the opposed walls and tank bottom at spaced apart intervals so that the size of the anode chamber or cathode chamber can be easily changed depending on which member is joined with a corresponding mating member to form the membrane holder.
In a further aspect of the invention a membrane holder for holding a membrane in an electrolytic cell having side walls and a bottom is provided comprising:
a female member comprising opposed vertical members and a connecting lower member, the members having openings for securing a bolt or other fastener, and preferably the openings are only partly therethrough;
a corresponding male member comprising corresponding opposed vertical members and a connecting lower member, the members sized to mate and form a watertight seal with the female member, with either the female member or male member being sized to form a watertight seal with the side walls and bottom of the cell, e.g., U-shaped for a rectangular cell, and having openings therein in the opposed vertical members and connecting lower member corresponding to the openings in the opposed vertical members and connecting lower member of the female member; and
wherein, in use, a membrane is secured between the female member and male member preferably with a gasket material and fasteners, e.g., bolts, extending in the openings in the male member and the female member openings to secure the membrane in the membrane holder.